patman: Support absolute and ~user-relative alias files

Python doesn't naturally support tilde (~) as a user-home marker in
paths, but git-config does. So we need to resolve it before continuing.

We also shouldn't blindly join the top-level tree with the aliasesfile
path, because it might be an absolute path.

This resolves warnings like the following:

  Warning: Cannot find alias file '/path/to/source/tree/~/.git-email'

Seen when git-config is like:

  $ git config sendemail.aliasesfile
  ~/.git-email

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Otavio Salvador <otavio@ossystems.com.br>